Salt and dust help unravel past climate change

Ice cores and climate change

Tiny amounts of salt and dust trapped in the Antarctic ice sheet for the last 740,000 years shed new light on changes to the Earth's climate.

The results, published this week in the journal Nature, come from the team who extracted a 3 km long ice core from Dome C, high on East Antarctica's plateau - the oldest continuous climate record obtained from ice cores so far.

Since reporting in 2004 that the Earth experienced eight climate cycles (each consisting of an ice age and warm period) the team have been analysing the chemical impurities in the cores to unravel how different parts of Earth's climate varied over the last 740,000 years. This work is vital for understanding future climate change.

By measuring the varying amount of salt in the cores the team can estimate how far the sea ice around Antarctica extended every time Antarctica got colder. The salt appears to come from brine expelled to the top of newly formed sea ice (frozen sea water). The white sea ice replaces the dark ocean, making the Earth reflect more sunlight.

Small dust particles are blown by the wind from surrounding continents. Many more of them are found in ice from cold times, and the team conclude that the nearest continent, southern South America, was much drier or windier. The extra dust may have provided nutrients to the ocean, helping microbes to take up CO2 from the atmosphere. From the different responses of salt and dust, the authors propose that each time the Earth warmed, emerging from an ice age, there was an order of events, with South America responding early, and sea ice extent responding late.

Lead author Dr Eric Wolff from British Antarctic Survey said,
"Our research shows that, throughout the last 740,000 years, every time cold conditions gave way to mild ones, similar changes occurred in the same sequence. We conclude that the Earth follows rules when climate changes and if we can understand those rules we can improve climate models and make better predictions for the future."

The Dome C drilling is part of the 'European Project for Ice Coring in Antarctica' (EPICA). The team at Dome C endured summer temperatures as low as minus 40ºC at the remote drilling site over a thousand kilometres from the nearest research station. The consortium completed the drilling in December 2004 after penetrating 3260 m of ice.

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Issued by the British Antarctic Survey on behalf of the EPICA chemistry consortium

The paper 'Southern Ocean sea-ice extent, productivity and iron flux over the past eight glacial cycles', is published in Nature on 23 March.